Conventionally, it is known that uncomfortable abnormal noise, i.e., so-called creep groan (or creep noise) is generated such that, during an operation of an automatic (AT) vehicle, while the vehicle is being stopped by a service brake, in a process of slightly loosening a brake pedal to completely release the brake, a torque is generated in a traveling direction due to creeping to transmit vibration generated by a change in relative speed between a brake pad and a rotor to a suspension system and a vehicle body so as to radiate the vibration. This creep groan is also generated in a manual (MT) vehicle, even when the brake is loosened while the vehicle is being stopped on a downhill slope with the service brake.
During generation of this creep groan, a stick-slip phenomenon occurs between the pad and the rotor. A reduction in energy for generating this stick slip is a major measure to suppress creep groan. Experientially, a reduction in difference between a static friction coefficient and a dynamic friction coefficient of the pad is one example of the countermeasures. The creep groan is likely to occur after rain, after leaving a vehicle unattended in the night, or in an atmosphere at a high absolute humidity and may also be caused by the influence of moisture. However, at present, the fundamental mechanism has not been known.
As a technique for solving the creep groan, for example, one described in Patent Literature 1 is known. In this technique, an amount of moisture present on the friction interface is adjusted by compounding a water absorbing material and a water repellent material (zeolite, antimony oxide, and fluorinated polymer) in the pad in a balanced manner so as to suppress the creep groan.
In addition, Patent Literature 2 discloses a friction material obtained by applying a raw material before molding with surface treatment using a silane-based water repellent material. Patent Literature 3 discloses a friction material containing a water-repellent fluorinated polymer. The techniques of these documents attempt to reduce noise by suppressing an increase in friction coefficient by suppressing moisture absorption and water absorption by a material having water repellency.